I'm an intern at a marketing department and got an assignment to update a banner with new information, and was directed to a pdf file to edit. It was only 3MB, which seemed very low for a large print-quality file (about 100 x 30 inches in size), but I opened it in Photoshop anyway (the original PSD and InDesign files are apparently misplaced).
I edit the image, flatten it back down without changing any settings (300 dpi, CMYK, the works), and suddenly I can't save a new file as a pdf since it's too large, almost 3 gigabytes. With no layers and exactly the same settings as the original pdf, I have no idea how to compress the file back down without losing quality. How was it only 3MB in the first place? I have no idea, and neither does anyone else, it seems.
Is there any way I can reduce the size without compromising the print quality? The lowest I can get to is 50MB after changing dpi to 150, but I'm not sure it's safe to tamper with that either.